As described in patent application Ser. No. 07/174,732, a common form of rotary sprinkler includes a nozzle outputting a jet axially of the nozzle, and a rotor rotatably mounted with respect to the nozzle in alignment with the axial jet and having a surface formation impinged by the axial jet and effective to deflect the jet laterally of the nozzle and to rotate the rotor. Such rotary sprinklers, however, have a tendency to rotate at a high velocity, which decreases the effective range of the sprinkler. For this reason, such sprinklers have been provided with a retarding device having a first surface rotatable with the rotor with respect to the nozzle, a second surface non-rotatable with respect to the nozzle, and a viscous liquid between the two surfaces for retarding the rotation of the rotor. Sprinklers equipped with such retarding devices are described in U.S. Pat. Nos. 4,660,766 and 4,796,811. In the sprinklers described in the above patents, the surface rotatable with the rotor with respect to the nozzle is in the form of a pin secured to the rotor, and the surface non-rotatable with respect to the nozzle is in the form of a cavity formed in a part of the rotary sprinkler fixed to the nozzle and containing the viscous liquid.